RootMetrics, a company that evaluates mobile network performance, tests carriers and ranks them on a scale from zero to 100. The semi-annual reports are conducted to determine the best carrier from a customer’s point of view.

Verizon retained its top rankings throughout 2014 in the categories of network reliability, speed, data, and call performance. However, it slipped to third in terms of text messages. AT&T, which preserved its No. 1 position for texting, remained in Verizon’s shadow as the second place winner in every other category.

While the top two carriers will surprise few, the real surprises came from the smaller carriers, Sprint and T-Mobile, which saw big improvements across all categories. The two “little guys” are beginning to close the gap on the mobile giants.

Ranking third for the second half of 2014, Sprint jumped ahead of T-Mobile in overall performance. The higher ranking was attributed to Sprint’s improvements in call consistency and network reliability.

While Sprint and T-Mobile may be closing the gap on performance, they have yet to be a real threat to the biggest US carriers. As Forbes reports, the smaller companies focus on highly competitive data plans and pricing, while the bigger guys put efforts into higher network quality and customers with a high "average revenue per user."

Though Verizon and AT&T will likely remain the biggest players for sometime, the introduction of alternative carrier services – such as Wi-Fi connectionplans and T-Mobile and Sprint lowering customer rates – means the two giants will probably need to adapt to the changing marketplace if they want to maintain their dominance of the industry.

In addition to new forms of competition, the quality over quantity approach has also left both carriers with large amounts of debt. AT&T and Verizon owe the Federal Communications Commission (FCC) a combined $29 billion after investing in the FCC's wireless spectrum auction and have been liquidating assets to cover bills. While both companies are still heavily investing in performance, the slip in finances could give others the chance to inch closer as serious rivals.

After ranking high above the competition again, there is little reason for AT&T and Verizon to switch up their tactics, but the mobile market is heating up, and consumers may begin to shun the giants for cheaper options.